High density electron source



IIA/iwf INVENTOR.

7'7'0/WVIY K. G. HERNQVIST Filed Dec. 26, 1957 HIGH DENSITY ELECTRON SOURCE May 17, 1960 wzl/W4- n l an? 7 4.1M ZM L." wrn. m, F/ M i. w W. M f.

KHRL E. HERNQVIST BY Wm /ffm HIGH DENSITY ELECTRON SOURCE Karl G. Hernqvist, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application December 26, 1957i, Serial No. 705,276 8 Claims. (Cl. 313-189) This invention relates to method and apparatus for producing a high density, high voltage electron beam and particularly to a method and apparatus for extracting a high density beam of electrons from an arc discharge plasma.- l

The desirability of high density electron beams for certain applications has long been recognized. However, electron emission limitations of available cathode materials and the problem of space charge limitations have Iprecluded the provision of such according to prior art practices. The possible uses of a high density electron beam are many, andinclude, for example, the application to klystron tubes, backward wave oscillators, andvmillimeter wave generation in gaseous plasmas. For example, in a conventional klystron a high density beam will permit higher power output with a smaller size aperture in thetube electrodes, and a smaller aperture will in turn result in a higher operating Q and improved coupling, which meanshigher efficiency. And in the generation of millimeter oscillations in gaseous plasmas, a high density electron beam is not just desirable but in fact necessary in order to initiate any oscillation.

It is therefore an object of my invention to provide novel and improved electron gun apparatus for producing' an extremely high density beam of electrons.

Another object of my invention is the provision of novel and improved electron gun apparatus for extracting a high density beam of electrons from a gaseous arc discharge plasma.

' Briey, according to my inventio-n, the plasma of an arc discharge having an anchored cathode spot is constrained'into a narrow column by a magnetic field. A

small aperture in the arc discharge anode is axially aligned with the cathode spot and communicates with a low pressure electron extraction chamber. The plasma ex. tends through the anode aperture into the low pressure chamber. Electrons are there extracted from the plasma and accelerated through a utilization drift space to a collector electrode. Means is provided for maintaining a low pressure in the electron extraction chamber.

In the drawings:

Fig. 1 is an elevation view in partial cross-section of kexperimental apparatus according to my invention;

.portion 12 and the anode 1-6 define an arc discharge chamber 18, and lthe second envelope portion x14 and the anode 16 define an electron extraction chamber 20.

'(Ihe anode 16 is provided with a central aperture 22 2,937,300 Patented May 17, 1960 ICC communicating between the two chambers 1'8 and 20. A mercury pool cathode 24 including a mercury wetted lanchor electrode 26 is disposed in the arc discharge chamber 18.

An electrode assembly 28 disposed in the electron extraction chamber 20 includes a control electrode 30, an accelerator electrode 32, first and second shield electrodes 34 and 36 respectively, and a collector electrode 38. 'I'he electrodes 30-36 of the electrode assembly 28 are each centrally apertured and are mutually axially aligned with the anode aperture 22 and the anchor electrode 26. As shownrin the drawing, electrodes 30 and 32 are located near the anode 16. The second shield electrode '36 comprises an'apertured disk 39 and a hollow cylindrical member 40 which extends toward the lirst shield electrode 3-4 to dene a utilization drift space 41 for an electron beam projected therethrough. The collector electrode 38 is provided with a recess 42 in its electron receiving surface to aid in the suppression of secondary electron emission.

A magnetic field indicated by the arrow H is provided parallel to and surrounding the line of this alignment by any conventional means, e.g., permanent magnet poles 43 and 44. v

A ilat, annular third envelope portion `46 is sealed between the anode 16 and the second envelope portion 14 and encloses an annular coolant chamber 48 which surrounds the electron extraction chamber 20. Liquid coolant such as Dry-Iced methanol is circulated through the cool-ant chamber 48 via an inlet pipe 50 and an outlet pipe 52. Such extreme cooling of the electron extraction chamber 20 results in a pressure differential between th'e two chambers 18 and 20 suihcient to permit the practical utilization of an electron beam in the electron extraction chamber Z0.

In operation of the experimental apparatus 10 of Fig. l, the mercury wetted anchor electrode 26 protrudes slightly Kabove the surface of the mercury pool 24 and serves to anchor the cathode spot of an arc discharge established between the cathode 24 and the anode 16. The axial magnetic eld H serves to constrain the plasma of the arc discharge into a narrow, dense column which imping'es upon the anode 16 over the aperture 22. Due to diffusion part of the plasma of the arc discharge extends through the aperture 22 into the electron extraction chamber 20 where its axial extent is limited by a high positive voltage on the nearby accelerator electrode 32. However," the high positive voltage on the accelerator electrode 32 serves to extract electrons from the plasma and accelerate them through the accelerator aperture and on through the drift space 41 to the collector electrode 38. Utilization apparatus (not shown), such as a klystron or backward wave oscillator apparatus can be disposed in the drift space 41 to utilize the projected high density beam of electrons passing therethrough.

In the presence of an arc discharge in the arc discharge chamber 18 a gas pressure exists therein of an approximately 0.1 mm. of Hg. The pressure in the electron extraction chamber 20 must be several orders of magnitude -lower so as to avoid excessive ionization and scattering of the electron beam. Since these two chambers communicate with each other via the connecting anode aperture 22, gas atoms and ions present .in the arc discharge chamber tend to diiuse through this aperture into the electron -extraction chamber 20 by circulating a coolantsuch as iced methanolthrough the annular coolant chamber 48.

principle of operation. These electrodes are incorporated.

in the apparatus because of their eifects on the electrostatic characteristics of the apparatus of obtaining better performance thereof. A description of the operation of the device 60 with a consideration of the eiects of these electrodes will be best understood with reference to Fig. 2.

In Fig. 2, relative spacingv of the electrode elements disposed between the pool cathode 24 and the collector electrode 38 of the experimental apparatus 10 is illustrated as in abscissa measurement on thev diagram. The approximate relative electrical potentials existing' along the pool-cathode-to-collector spacing is represented by the curve 55 as an ordinate measurement.

As described above, according to the basic principle of operation of my invention an arc discharge is established between the pool cathode 24 and the anode 16 and extends through the anode aperture 22 into the electron extraction chamber 20. As shown in Fig. 2, the plasma column also extends axially through the control electrode aperture for a short distance. One of the purposes ofthe control electrode 30 is to fix the axial extent of the plasma column. The amount of this extent is illustrated in Fig. 2 by the plane P. It can be seen from Fig. 2 that the plasma potential is very nearly equal to the anodeV potential. Thus, no electrostatic force is presentedT to the plasma by the anode which would otherwise prevent diffusion of the plasma through the anode aperture 22. If, as shown, the control electrode 30 is operated slightly negative with respect to' the plasma, a virtual cathodev is formed at the plane P and serves as the emission surface for the electron beam produced. This insures that the electron beam extracted from the plasma is always space charge limited.

Since the control electrode does fix the position of the plane P, its presence is considered highlyadvantageous in that it is believed to be of somewhat importance to maintain the distance d (Fig. 2) constant. This stems from the theory that if the extent of the plasma, and hence the'distance d, is permitted to uctuate, an undesirable noise factor is introduced into the high density' electron beam.

If so desired, the control electrode 30 can also serve to modulate the electron beam extracted from the plasma. This feature of the control electrode 30 can be Vof special significance in that it makes possible compensation for the instabilities inherent with arc discharges. Stated otherwise, since theV control electrode can exert a modulation effect upon the electron beam, the effect of plasma variations of an arc discharge can be neutralized by feedback from the collector electrode 38 tothe controll electrode 30. The controlelectrode 30 also serves the'function of an ion collector for those positive ions diffusing through the anode aperture.

Still referring to Fig. 2, the accelerator electrode 32 serves to extract electrons from the plasma and accelerate them to the collector electrode 38 and also to repel positive ions in the plasma. The twol shield electrodes 34 and 36 incorporated in the experimental apparatus 10 serve to shield the drift space 41 and to there provide ion trapping in order to insure space change neutralization of the electron beam. These shield electrodes are operated slightly negative with respect to the accelerator and collector' electrodes.` The-fionftrappig of 'the shield elec- 4` trodes 34 and'36 isconventional and is discussed by Field, Spangenberg, and Helm in Electrical Communications, vol. 24, No. 1, March 1947.

Referring to the potential curve 55 of Fig. 2, approximate voltages along the axis of the apparatus 10 from the pool cathode 24 to the collector electrode 38 are plotted relative to a zero potential for the anode 16. The pool cathode 24 and control electrode 30 are shown to be operated only slightly negative, the accelerator electrode 32 and collector electrode 3S at the highest positive voltage applied to the apparatus, and the shield electrodes 34 ard 36 only slightly less positive than electrodes. 32 and 3 Most of the dimensions ofthe experimental apparatus i0 are not critical. However, special consideration should be given to the size of the anode aperture 22 and the spacing of the control electrode 30 therefrom. As will be appreciated, operation of an arc discharge in the are discharge chamber 18 results in a pressure therein which is far in excess of that which is tolerable in the electron extraction chamber 20. It is for this reason that the pressure differential means must be provided. Likewise, it is apparent that by virtue of the differences of the pressures in the 4arc discharge chamber 18 and the elect-ron extraction chamber 20, gas ions and neutral gas atoms will tend to diffuse from the arc discharge chamber through the anode aperture 22 into the electron extraction chamber. As previously described; the extent of diffusion of gas ions passing through the anode aperture 22 will be limited by the high positive charge on the accelerator electrode 32 and controlled by the negatively biased control electrode 30. Neutral gas atoms passing through the anode aperture will not be subjected to any such electrostatic forces, and their diffusion into the electron extraction chamber 20 will thus be electrostatically uninhibited. For this reason the size of the anode aperture 22 is a main factor determinative of the amount of gaseous diffusion therethrough. The localized gas pressure in the vicinity of the electron extraction region due to gas diffusion through the anode aperture would of course be present even if the background pressure in the electron extraction chamber were zero. If the control-electrode-anode--aperture spacing is too short, the gas atoms passing through the anode aperture will not have a chance to diffuse suiciently and will consequently present a gas pressure immediately adjacent the electron extraction region at plane P which is in excess of that permissible for satisfactory electron extraction.

The following formula mathematically states the relationship of the size of the anode aperture to the spacing between this aperture and the virtual cathode plane `P.

Here, AB is the area of the beam cross-section, that is, the area of the anode aperture; Lo is the distance between the plane P and the anode; p3 is the gas pressure inthe electron extraction region; and pn is the gas pressure in the `arc discharge region. In the experimental apparatus 10, pD=0.l mm. of Hg, p=2 l0-4 mm. of Hg, and an anode aperture diameter of 0.013 inch yields L0=`l.7 mm.

In establishing the location of the plane P relative to the accelerator electrode 32 the following formula can he applied. In it the distance d (Fig. 2) in' centimeters between the plane P and the accelerator electrode 32 is determined by the 3/ 2 law for space charge limited electron flow in a diode.

where VL is the acceleration voltage and]e is the electron current density in amperes/cm.2.

The spacing between the anchor electrode 26 andthe anode aperture 22, although-not critical, must nevertheless `be considered. One characteristic of an arc discharge is that gas pressure is greatest at the cathode spot and decreases with distance therefrom. If the anchorelectrode-anode-aperture spacing is shortened, the gas pressure in the arc discharge chamber adjacent the anode aperture is, of course, raised. Such an increase of pressure causes an increase in gaseous atom diffusion through the aperture and the attendant problems thereof. 'Hence, it is apparent that a minimum spacing is established. On the other hand, if this spacing is made too great, the magnetically constrained column of plasma expands notwithstanding the effect of the magnetic field. As a result, space charge limitations of the electrons in the arc discharge becomes a problem. From this it follows that a maximum spacing is established. For the experimental apparatus this minimum and maximum spacing is established at approximately 100 and 200 mils respectively. Y

Generally speaking, the spacing and general dimensions ofthe other electrodes of the experimental apparatus 10 are not at all critical. In the experimental apparatus 10, the spacing between'the control electrode 30 and the -accelerator electrode 32 was made 40 mils; likewise, the spacing between the accelerator electrode 32 and the first shield electrode 34 was made 40 mils. The spacing between the shield electrodes 34 and 36 depends upon the amount of space desired for inclusion of utilization apparatus. No special limitation is placed upon this spacing save those of conventional design as known to the prior art. I

Regarding the aperture size of the four electrodes 30, 32, 34, and 36 this too is not considered to be critical. In the experimental apparatus 10 the apertures ot' these four electrodes were made 13 mils, as was the anode aperture 22. However, if desired, in order to minimize electron impingement upon these electrodes their apertures might be made slightly larger than the anode aperture.

Regarding the thickness of the four apertured electrodes, 30-36, the most important consideration is that they be as thin as possible yet massive enough to conduct away the heat which is generated therein. In the experimentalv apparatus 10 these lfour electrodes were made of 40 mil thick material. However, the anode is subjected to much more heating, and although the thickness plays no material role as to electrostatcs involved, it Vshould be sufficient to handle the heat generated therein. In the experimental apparatus 10 this was approximately 0.125 inch. The presence of such excessive heating of the anode is realized when we consider that a plasma column 50 mils in diameter is developed by virtue of a 50 mil diameter anchorele'ctrode 26. But since the anode aperture 22 is only 13 mils, less than 7% of the plasma column cross-section fails to impinge upon the anode 16.

Consideration should alsobe given to heat dissipation in the collector electrode 38 since an extremely high density beam impinges thereon. Localized heating of this electrode can be lessened by causing the beam to expand immediately prior to impingement and consequently distribute the heat generated thereby over a larger area. This can be accomplished by providing the recess 42 in the electron receiving surface of this electrode and making the electrode of magnetically conductive material. This will result in a divergence of magnetic lines of force entering the recess 42 and cause theV electron beamjto also diverge, or expand. Y 1

In operation of the experimental apparatus 10, voltages were applied to the various electrodes as follows:

kReferring to Fig. 3, a co'mmercial typevembodiment of I electron discharge device according to my invention is shown. The device 60 is similar to the apparatus 10 of Fig. `l in that an arc discharge chamber 62 and an electron extraction chamber 64 separated by an anode 66 having a communicating aperture 68 is provided. Also, as in the apparatus 10, the device 60 includes a mercury pool cathode 70 and a mercury wetted anchor electrode 72 asso'ciated therewith disposedin the arc discharge chamber 62. However, the electron discharge device 60 is illustrated as incorporating conventional backward wave oscillator apparatus 74 in its electron beam utilization drift space 76.

Ihe physical envelope structure of the device 60 also differs in that the anode 66 comprises a major portion of the enclosing wall of the electron extraction chamber .64. Cooling to provide differential pressure for the device 60 comprises a coil 78 wrapped tightly around the wall .of the electron extraction chamber 64 and soldered thereto. A fluid coolant such as Dry-Iced methanol is circulated therethrough. A pair of magnet poles 80 and 82 pro'vides an axial magnetic field H.

The electrode arrangement in the electron extraction chamber 64 comprises 'a control electrode 84, an accel- V erator electrode 86, first and second shield electrodes 88 and 90 respectively and a collector electrode 92. The electrodes 84-90 are each centrally apertured and aligned with the anode aperture 68 and the ancho'r electrode 72. In orderl that a more sensitive control of the electron beam may be obtained, these electrodesespecially the control electrode-may include a grid structure overlying the apertures thereof. Such grid structure may comprise merely a single cross of fine wire. The first shield electrode 88 also serves as one wall of a wave guide 94 which makes up part of the backward wave oscillator apparatus 74. The shield electrodes include a conductive hollow cylindrical member extending therebetween and defining the drift utilization space 76. A self-supporting helix conductor 96 electrically connected between the rst and second shield electrodes 88 and 90 and extending transversely through the waveguide 94 completes the apparatus 74. 'I'he helix 96 is disposed along the line of axial alignment of the anchor electrode 72 land the anode aperture 68. 'I'he wave guide 94 extends to a-plane adjacent to a window 98 in the envelope wall of the electron extraction chamber 64 for transmission of energy therethrough.

Apparatus according to my 'invention has been illustrated as taking certain specific fo'rms. However, certain alternative structure can .be used without departing from the spirit of my invention. For example, a hot cathode arc discharge rather than the cold cathode mercury pool arc discharge can Vbe used. Likewise, ionizable gases o'ther than mercury can be used, for example, xenon. Also, as briey mentioned, means 'for producing a pressure differential between the arc discharge and electron extraction chambers need not take the form illustrated. Continuous pumping of the electron extraction chamber without cool- Aing is an entirely satisfactory alternative to the extreme ,cooling thereof. These three possible alternatives represent only 4a few of the possible modifications that will be readily suggested to one skilled in the art.

The requirement of fixing the location of the cathode spot of the arc discharge in the practice of my'invention has been illustrated vbythe use of a mercury wetted anchor electrode 26 disposed in, and protruding slightly Kabo've, the mercury pool 24. Such an arrangement is known in the prior art. An alternative device for fixing the cathode spot of the arc discharge could take the form of a small mercury pool of size comparable to the electrode 26 shown.

What is claimed is:

1. Apparatus for producing a high density beam of electro'ns comprising a iirst chamber and a second chamber separated by a conductive member having an aperture therethrough communicating between said chambers; means in said iirst chamber for establishing an arc discharge; means in said iirst chamber for xing the position of the cathode spot of said arc discharge; electrode means in said second chamber having an aperture adjacent to said member for accelerating a beam of electrons away from said aperture; said cathode spo't iixing means, said aperture in said conductive member, and said aperture in said electrode means being in mutual alignment; means for providing a magnetic field parallel to and extending along the axis of said alignment between said cathode spot fixing means and said aperture Vin said conductive member; and means adapted to maintain a substantially lower pressure in said second chamber than in said rst chamber.

2. Apparatus for producing a high density beam of electrons comprising an arc discharge chamber and an electron extraction chamber separated by a wall member having an aperture therethrough communicating between said chambers; means including a cathode having portions of limited area for establishing an Varc discharge plasma extending through said aperture; means including an electron accelerator electrode in said electron extraction chamber having an aperture adjacent to said wall member for extracting electrons from said plasma and accelerating them away from said aperture; the aperture of said electrode being in alignment with said wall member aperture and said cathode portion of limited area, means producing a magnetic force extending coincident with the axis of said alignment between said cathode portion of limited area and said wall member aperture; and means providing a low pressure in said electro'n extraction chamber relative to the pressure in said arc discharge chamber.

3`. Apparatus for producing a high density electron beam comprising an arc discharge chamber containing a mercury pool cathode including a mercury wetted anchor electrode; an electron extraction chamber; an anode cooperating with said mercury poo'l cathode comprising a wall member separating said arc discharge chamber and said electron extraction chamber; said wall member having an aperture therethroughcommunicating between said chambers; a disk electronv accelerator electrode mounted in said electron extraction chamber and having an aperture adjacent to said wall member; the aperture of said accelerator electrode; said wall member aperture, and said anchor electrode being in mutual axial alignment; means for pro'viding a magnetic eld extending coincident with the axis of said alignment between said anchor electrode and said wall member aperture; and means for maintaining a substantially lower gas pressure in said electron extraction chamber than exists-in said arc discharge chamber. Y

4. Apparatus for producing a high density electron beam comprising an arc discharge chamber containing a mercury pool cathode including a mercury wetted anchor electrode; an electron extractio'n chamber; an anode cooperating with said mercury pool cathode comprising'a wall member separating said arc discharge `chamber and said electron extraction chamber; said wall'anode having an' aperture therethrough communicating between said chambers; a disk electron accelerator electrode mounted in said electron discharge chamber and having an aperture adjacent to said wallV member; the aperture offsaid accelerator electrode, said Wall anode aperture, and-said anchor electrode being in mutual axial alignment; means for providing a magneticy eld coincident with said align.-

ment and extending between said anchor electrode and said anode aperture; and cooling means for maintaining a substantially lower gas pressure in said electron extraction chamber than exists in said arc discharge chamber; said cooling means comprising an annular chamber surrounding said electron extraction chamber and inlet and outlet pipes for circulating uid coolant therethrough.

5. Apparatus for generating a high density electron beam comprising an arc discharge chamber and an electron extraction chamber, said chambers being separated by a conductive member having an aperture communicating between said chambers, means for establishing an arc discharge in said discharge chamber, means fixing the cathode spot of said arc discharge in a predetermined location adjacent said aperture, magnetic means providing a magnetic eld extending between and along aV path dei-ined by said= aperture of said conductive memberand said predetermined location for constraining the plasma of said arc discharge into a narrow column extending through said aperture, apertured electrode means disposed in said electron extraction chamber adjacent to said conductive member fo'r extracting electrons from said extending plasma and accelerating them away from said aperture, electrode means intermediate said last-named means and said conductive member for fixing the amount of extent of said extending plasma, and means for maintaining a substantially lower pressure in said electron extraction chamber than in said arc discharge chamber.

6. Apparatus for producing a high density beam of electrons comprising a iirst chamber and a second chamber separated by a conductive member having an aperture therethrough communicating between said chambers; means in said: first chamber for establishing an arc discharge; means in said rst chamber for xing the position ofthe cathode spot of said arc discharge; apertured electrode means in' said second chamber adjacent to said conductive member for accelerating a beam of electrons; electrode means intermediate said conductive member aperture and said accelerating electrode means fo'r modulating said beam of electrons, electrode means in said second chamber spaced from said accelerating electrode means for collecting said beam of electrons,shield means extending substantially between said accelerating means and said collecting means defining an electron beam utilization drift space, means providing a magnetic eld extending between and along the path defined by said cathode spot ixing means and said conductive member aperture, and means adapted to' maintain a substantially lower pressure in said second chamber than in said first chamber. Y

7. Apparatus for producing a high density beam of electrons comprising an arc discharge chamber and an electron extraction chamber separated by a wall member having an aperture therethrough communicating between said chambers; means including a cathode having portions of limited area for establishing an arc discharge plasma; an electrode assembly disposed in said electron extraction chamber, said assembly comprising an apertured electron modulator electrode, an apertured electron accelerator electrode, apertured rst and second shield electrodes, and an electron collector electrode spaced in the order named from said wall member, said modulator and accelerator electrodes being adjacent to said wall member, the apertures of said apertured electrodes being in mutual alignment with said wall member aperture and said portion of limited area of said cathode, at least one of said shield electrodes including a hollow cylindrical member extending toward the other of said shield electrodes, means producing a magnetic eld coincident with the line of said alignment and extending between said cathode and said wall member; and means providing a substantially low pressure in said electron Vextraction cham- Yber relative to the pressure in said arc discharge chamber.

' 8. Apparatus for producing a high density electron beam comprising an arc discharge chamber; a mercury 1 aca'asoor pool cathode including a mercury wetted anchor electrode disposed in said arc discharge chamber; an electron extraction chamber; an anode co-operating with said mercury pool cathode comprising a wall member separating said arc discharge chamber and said electron extraction chamber; said anode having an aperture therethrough communicating between said chambers; an electron collector electrode mounted in said electron extraction chamber; apertured disk electron modulator, accelerator, Y

and first and second shield electrodes disposed in the order named from said anode to said collector electrode; said modulator and accelerator electrodes being adjacent to said anode; said anchor electrode and the apertures of said anode, modulator, accelerator, and shield electrodes being in mutual alignment; atleast one of said shield electrodes including a hollow cylindrical member electrically connected thereto and extending substantially to the other of said shield electrodes coaxially with the axis of said alignment and defining an electron drift space; means providing a magnetic eld coincident with the axis of i said alignment and extending betweenL said'anchor electrode and said anode; and cooling means for maintaining a substantially lower gas pressure in said electron extractionV chamber than exists in said arc discharge chamber, said cooling means comprising a iluid conductor coil disposed around said electron extraction chamber and having an inlet and outlet for circulating iluid coolant therethrough. t

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